Carboplatin is a known cytostatic agent. Up till now, this agent was marketed as a freeze-dried product and accordingly it is reconstituted before administration. Of course, a ready to use solution would be a much more advantageous composition, but carboplatin is not stable in a simple aqueous solution. As has appeared by research, carboplatin can decompose through various mechanisms, whereby a.o. the highly poisonous compounds di-.mu.-hydroxo-bis(cis-diammineplatinum (II)), and tri-.mu.-hydroxo-tris(cis-diammineplatinum (II)) can be formed.